In production machines that work with a web of material that is unwound from a roll, such as is the case in e.g. rotary printing presses, the unwinding roll is arranged in a roller stand, e.g. mounted on a shaft and provided with a braking mechanism which is arranged on the shaft or on the roll periphery and adjusted to allow control of the tension in the web part extending between the roll and the production machine. When the roll is exhausted, the whole production machine is stopped and fitted with a new roll, whereafter the machine may again be set in motion.
In the case of high-speed production machines, roller stands with means for automatic roll shifting are used to increase the machine output and to minimize the effects of disturbances to the production process, e.g. to the drying of printing ink, etc.
Roller stands of this kind are provided with at least two attachment points for the rolls, whereby space is provided for one unwinding roll as well as for one new roll. The roller stand with means for automatic roll shifting is arranged in such a manner that when the web of the unwinding roll is nearly exhausted, the new roll will automatically, or through a push-button signal, be brought to rotate while driven either from the centre or by drive belts abutting against the roll periphery. The leading end of the web on the new roll adheres lightly to the underlying turn of the web and on its upper face a coat of glue is applied (or e.g. a tape which is adhesive on both sides). In the area of the joint between the web of the unwinding roll and that of the new roll the paper webs are brought into contact with one another with the aid of a brush and/or a roller and once the leading end of the web of the new roll is glued to the unwinding web the function of a severing knife is initiated to cut off the previously unwinding roll. The web of the new roll is now joined to the paper web.
To ensure that the above-mentioned system will function, careful synchronization is required between the speed of the running web and the peripheral speed of the new roll. This is achieved e.g. with the aid of electronic comparator systems, whereby the peripheral speed of the new roll is measured by a tachometer generator the signal of which is compared with a corresponding signal issued by a generator mounted on e.g. the main shaft of the production machine. With the aid of other automatic means the peripheral speed of the new roll is to be brought to coincide with that of the unwinding web.
To enable continuous unwinding of the web of material the roller stand must be equipped with two unwinding stations. Consequently, considerable floor space is required for the roller stand. In addition, the roller stand becomes complex because as a rule one braking mechanism is required for each unwinding station to retain the web tension at the required level.